Solubility is believed to occur between materials, for example, a solvent and a polymer, when the Gibbs free energy of mixing is less than zero. The energy of mixing is discussed by Dr. Billmeyer in the "Textbook Of Polymer Science", Second Edition, John Wiley and Sons, Inc. (1975), pages 24-26, the disclosure of which is incorporated herein by reference. As explained by Dr. Billmeyer, the free energy of mixing .DELTA.G is defined according to the following equation: EQU .DELTA.G=.DELTA.H-T.DELTA.S,
wherein .DELTA.H represents the enthalpy of mixing, .DELTA.S represents the entropy of mixing, and T is the absolute temperature.
For reasonably nonpolar molecules, and where the degree of hydrogen bonding is insignificant or nonexistent, the enthalpy of mixing is positive and can be derived from the following equation: EQU .DELTA.H=V.sub.1 V.sub.2 (.delta..sub.1 -.delta..sub.2).sup.2,
wherein V is the volume fraction and subscripts 1 and 2 refer to the solvent and the polymer, respectively. The quantity .delta..sup.2 is the cohesive energy density of an ingredient. The quantity .delta., which represents the square root of the cohesive energy density, is known as the Solubility Parameter.
Generally, in the absence of significant hydrogen bonding, solubility can be expected to occur between a solvent and a polymer if the difference in Solubility Parameters (.delta..sub.1 -.delta..sub.2) is less than about 1.7-2.0. In other words, materials having similar Solubility Parameters tend to reach thermodynamic equilibrium when they are mixed together, and this tendency toward equilibrium causes the different molecules to attract and mix with each other. Materials having dissimilar Solubility Parameters tend to reach thermodynamic equilibrium when separated, and this causes the different molecules to repel and separate from each other.
The Solubility Parameters of different substances have been the subject of various calculations and publications. For a polymer, a relatively easy way to determine .delta..sub.2 is from a summation of molar attraction constants multiplied by the density and divided by the repeating unit molecular weight: ##EQU1## wherein .SIGMA. is the summation operator, E.sub.i is the molar attraction constant for each chemical moiety making up the repeating unit, M. W. is the molecular weight of the repeating unit, and .rho. is the polymer density.
The Solubility Parameter (.delta..sub.1 or .delta..sub.2) can be expressed in Solubility Parameter Units (S.P.U.'s), with one S.P.U. being equal to one [J/m.sup.3 ].sup.1/2 .times.10.sup.-3, where J=Joules and m=meters. For a more detailed explanation of the Solubility Parameter and its values for various compounds, see the Third Edition of "Polymer Handbook", edited by Bandrup and Immergut, and published by John Wiley and Sons, Inc., New York, N.Y., in 1989. The chapter of the "Polymer Handbook", beginning on page VII/519, and entitled "Solubility Parameters", is incorporated herein by reference.
Fruit-flavored chewing gums (whether they be wax-containing or wax-free) have a well-known characteristic or problem of rapid flavor release and short flavor duration. Fruit-flavored chewing gums have a tendency to exhibit a very rapid flavor release in the early stages of chewing, followed by rapid dissipation until the fruit flavor becomes virtually undetectable after 8-12 minutes of chewing.
This problem has been addressed in the prior art by using certain encapsulation techniques for slowing the release of fruit flavors. However, these encapsulation techniques present an additional problem in that they operate by creating a physical barrier to modify the release of fruit flavors. Also, the pleasing initial flavor burst associated with fruit-flavored gum is, to some extent, sacrificed when these encapsulation techniques are used. Additionally, it is known that a large percentage of the fruit flavor initially added to the chewing gum (in some cases, up to 60%) is never actually tasted by the consumer.
In order to provide a pleasing long-lasting flavor to a free fruit flavored chewing gum, it is generally desirable to prolong the time period during which at least 15% of the original flavor intensity is apparent to the consumer. In other words, the consumer initially detects a flavor burst of a relatively high intensity when chewing is commenced. Thereafter, the detected flavor intensity inevitably declines with time during chewing. When the detected flavor intensity decreases by more than 85% from the original level, the gum becomes annoyingly low in taste and is less pleasant to chew. Therefore, the enjoyable chewing time can be prolonged by prolonging the time during which at least 15% of the original flavor intensity is apparent.
In order to preserve the initial flavor burst and maximize the efficient use of the flavor ingredients, it is desirable that as much of the fruit flavor as possible be in the free (unencapsulated) form. Therefore, there is a considerable demand for a technique for enhancing the long-term detectability of fruit flavors without relying significantly on conventional encapsulation techniques.